The per-order limit for Press Sheets was lifted because it was essentially unenforceable. If one wanted to corner the market on Press Sheet limits, all they had to do was place multiple orders using a different name (wife, child, mother, brother, second cousin ... whatever) and/or send some to a business address and others to a home address, etc.
Further, the USPS does not really care about collectors. They are in it for the value they receive in selling these Press Sheets (which for the most part are retained as collectibles rather than used for postage) which is pure profit for the USPS.
Given the vast quantity of new issues coming out, I think the USPS is learning that they are better off creating only a limited number of Press Sheets and selling them right away as compared to issuing a larger number only to have them remain in stock until they become obsolete and ultimately get destroyed.
There have also been reports that some Press Sheets are made available at First Day of Issue ceremonies and collectors and/or speculators buy up large quantities there, too.
I wouldn't hold too much confidence in the numbers shown on the USPS
ebay listings, as I have seen numbers change for apparently no valid reason. Furthermore, that is only a small part of their sales when considering those orders made at the FDOI ceremony or those calling the 800# for an order, or those ordering online through the USPS store's website.
With "only" 500 Press Sheets available -- which comes down to only 10 Press Sheets for collectors in every US State -- that's a very small number and it is only common sense to expect that they would sell out very quickly.